Thanks stringy. It turns out that I didn't have as clean of fit in the scupper as it appeared. On my boat the top and bottom of the scupper halves are misaligned just enough to break the valve off. A smaller valve is needed to get a clean fit -- one that can be adopted to fit inside the scupper cap would be ideal. This could also be a floating ball valve.

For newer members, any kind of tape works for the venturi slot, but a clear packaging tape leaves no residue, has minimal stretch and good drag characteristics. In comparing tapes, the premium tapes last longer -- at least a couple of months if you're careful. Differences are small, so whatever you have around is fine. If you're going out to get some, 3M Scotch 3850 has the best adhesion and tensile strength, but Hinkle (Locktite) HP260 gives a lot of bang for the buck IMO. I use both.

For kayaking, you can also tape up the daggerboard well (AI) (keep the venturi slot) to eliminate water intrusion and improve speed. You can insert the board right through the tape any time. Taping the centerboard well on the TI while kayaking also improves speed.

I was under the impression that all scupper holes have a built-in Bernoulli effect of their own, hence one reason they are found on kayaks. While in motion, the Bernoulli vacuum pulls water from the cockpit.

So why tape the scuppers? If a larger wing area creates more lift, would not a larger scupper (and more speed) draw water more efficiently?

Or are we talking a "venturi" effect here, where narrowing an opening creates an acceleration of air/fluid which is already under pressure? Sort of like my snoring.

Stainless turnbuckles are used to attach and create tension at rear and I am now using cord and jam cleats to secure at the front and sides. Its very secure, very comfortable, much drier, and doubles as a perfect camping seat, ideal for expedition and overnighter use.

I'll update this post with photos of the cord/cleat system I'm now using when I get some photos on the water this weekend.

was under the impression that all scupper holes have a built-in Bernoulli effect of their own, hence one reason they are found on kayaks. While in motion, the Bernoulli vacuum pulls water from the cockpit.

Usually, but not automatically. For instance, the daggerboard well, CB well and drivewell all scoop water in. Scuppers having a teardrop recess behind them generally have a positive Bernoulli effect.

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So why tape the scuppers? If a larger wing area creates more lift, would not a larger scupper (and more speed) draw water more efficiently?

Tape over the front part of the opening gives a huge boost to the venturi (when going forward). This is nothing new. You may remember the old vent wings on automobiles (that could also reverse and become ram ducts), venturi vents on the roofs of RVs, sunroofs and vent visors on cars. This isn't really about lift.

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Or are we talking a "venturi" effect here, where narrowing an opening creates an acceleration of air/fluid which is already under pressure?

No, the boost comes from aligning the exit flow with the slipstream cleanly rather than dumping it. This also reduces drag. Keep in mind, if the boat goes backwards the venturi "vacuum" becomes a ram and the seat area will flood.

To demonstrate this powerful effect, hold a limp piece of letter sized paper with both hands up to your mouth, blow over the top of it (creating a lower pressure slipstream) and watch it lift the paper!